The MERMAID Project | The Son-O-Mermaid

In the last few decades seismologists have mapped out elastic
wavespeeds of the Earth's interior with often perplexing if not always
uncontested detail. Earthquake sources used in seismic tomography lie
mostly on plate boundaries; receivers mostly on dry land. The uneven
coverage resulting from this fundamentally inadequate source-station
distribution leaves large volumes inside the Earth entirely unsampled.
Placing seismic stations on the ocean bottom is often touted as the
only solution. The MERMAID project (Mobile Earthquake Recorder in
Marine Areas by Independent Divers) is a radical low-cost alternative
that uses passively drifting autonomous hydrophones with a now proven
potential to record hundreds of distant earthquakes over their
projected life span. The Son-O-Mermaid
prototype is a grounds-up
design that aims to alleviate some of the shortcomings of MERMAID.

Introduction

Earth models in which seismic wavespeeds vary only with depth are
sufficiently well constrained to accurately locate earthquakes and
calculate the paths followed by seismic rays. The differences between
theoretical predictions of seismograms in such one-dimensional Earth
models and observations can be used to reconstruct the
three-dimensional (3D) wavespeed distribution of the Earth in the
regions sampled by the seismic waves.

Caused by thermal and compositional variations, wavespeed anomalies
remain the premier observable to fully understand the structure and
evolution of our planet, from the scale of mantle convection and the
mechanisms of heat transfer from core to surface to the interaction
between the deep Earth and surface processes such as plate motion and
crustal deformation.

Limitations of Global Tomography

Unequal geographical data coverage remains the fundamental limiting
factor in seismic tomography. Only at great cost can geophysicists
overcome the difficulties of placing seismographs on the two thirds of
the Earth's surface that is covered by oceans, which strongly hampers
the determination of the structure of the Earth in the uncovered
regions. Thus, all 3D Earth models are marked by blank spots in areas
where little or no information can be obtained.

Remediating this problem requires the observation of seismic waves in
the oceans. Sonobuoys have had success in the past in recording local
earthquake signals, but have been too noisy to give an acceptable
signal-to-noise ratio for all but the strongest earthquakes. Ocean
bottom seismometers (OBS) and moored hydrophones are capable of
addressing the coverage gap, but they are expensive to manufacture
(about $50,000 for a three-component OBS) and deploy (about
$40,000 per day of ship time). Unable to communicate, stationary
underwater devices have to be retrieved at regular intervals.

Design of a mobile receiver

Oceanographers have designed robotic floats that spend their life at
depth but surface periodically -- using a pump and bladder -- to make
temperature and salinity profile measurements. Such low-cost
(about $15,000) Sounding Oceanographic Lagrangian Observers (SOLO)
can be equipped with a hydrophone to record water pressure variations
induced by compressional (P) waves. Untethered and passively
drifting, such a floating seismometer will surface upon detection of a
useful (for global tomography) seismic event, determine a GPS location
and transmit the waveforms to a satellite. The surfacing speed
guarantees a location accuracy of the float at depth to within a few
hundred meters. Operating costs are minimal: their autonomy and low
weight guarantee easy deployment from any vessel, and the data will be
available in real time; what's left is the price of a satellite
subscription.

The design challenges are formidable because, pending alternative
means of power generation, the success of the device depends on how
long it can last before its batteries run out or corrosion and
barnacles take over. Lifespan is critically dependent on limiting
power consumption by using a minimum of numerical operations to
perform the detection and identification of the waveforms. Recent
advances in signal processing have allowed us to address this
bottleneck: our tests have demonstrated the success of so-called
second-generation wavelets to provide useful sensitivity and
discriminating power, even in the presence of high levels of
contaminating noise.

Proof of concept

The great promise of this technology was demonstrated by the prototype
on its maiden voyage (November 4-6, 2003. A second test was conducted
September 10-12, 2004, and a third test on August 9-11,
2007). Submerged, and freely drifting for about 30 hours, 700 m below
the sea surface, in a canyon off the coast of La Jolla, California,
MERMAID recorded a very promising signal, coming from a relatively
faint (in global seismological terms) magnitude 6 earthquake near the
west coast of Colombia, about 5,000 km away. Earthquakes of a
magnitude larger than this occur at a rate of about 200 per year. The
recording shows a clear incoming P wave whose precise arrival
time can be determined to within a fraction of a second. The
demonstrated high sensitivity of the MERMAID platform clearly
illustrates its likely contributions to global seismic
tomography.

In addition to recording teleseismic P waves, such a system will
pick up trapped T waves propagating in the SOFAR (Sound Fixing
And Ranging) channel. Although noise for the purposes of global
tomography, recent hydroacoustic studies have shown their utility in
studying the mechanisms of large, tsunamigenic earthquakes such as the
Dec. 26, 2004, Sumatran earthquake.

Recently, Guust Nolet and his team have reported exciting new
successes with an improved Mermaid prototype. These developments were
described in an EOS article that can be found at the top of this page.

The future

A worldwide array of MERMAID floating hydrophones, on the scale of the
current international land-based seismic arrays, has the potential to
progressively eliminate the discrepancies in spatial coverage
resulting in poorly resolved seismic Earth models. Oceanographers have
already pointed the way with the large-scale international Argo project. There are currently
over 3,000 SOLO floats measuring conductivity,
temperature and depth throughout the Earth's oceans, to understand and
forecast climate. Added to future generations of the Argo project,
MERMAID's regular resurfacings will provide useful corollaries to
other disciplines, such as average current speeds at depth, spot depth
soundings, and, with the ongoing miniaturization of marine technology,
an additional payload of low-power instruments only limited by the
imagination.

Many of the important dynamic processes in the deep Earth seem now
located beneath the larger oceans in the southern hemisphere. This may
not be accidental, but the absence of seismic observations in the
southern oceans severely limits our ability to study these processes.
Does the Earth's mantle convect as a whole or is it layered? What is
the contribution of mantle plumes to the transport of heat to the
Earth's surface? What is the scales of mantle heterogeneity and how
does it originate? What are the nature and role of geochemical
reservoirs? Is there an undifferentiated reservoir in the lowermost
mantle?

Seismic tomography will provide the primary models in an Earth systems
framework ultimately involving geodynamics and geochemistry. But,
first and foremost, definitive answers will lie in the data: currently
hidden in plain sight, out of reach of more conventional approaches.

Son-O-Mermaid

Bud Vincent from the
University of Rhode Island and I have proposed to develop and build a
low-cost geophysical instrument with the same long-term goal of
closing the coverage gap between continental and oceanic data
collection, but one with a much expanded versatility, wider range, and
longer life-span than Mermaid. Our Son-O-Mermaid, depicted
below, will be an instrument (collecting earthquake data much
like a sonobuoy listens to submarines as well as making measurements
of ocean floor depth and providing basic current speed data) as much
as an instrument platform. The hurdles that need to be overcome
and the problems solved in order to make this instrument be of use for
seismology, most specifically, set a very high bar in terms of energy
efficiency, instrument accuracy, and longevity, and as a result,
future generations of it should be easily adapted to less demanding
data collection exercises (be they physical, chemical, or
biological).

Money

Despite the successes of Mermaid and its early support
($99,886) by the U.
S. National Foundation, funding for the Son-O-Mermaid
has been hard to come by, and we are currently operating on a
shoestring budget while Guust Nolet is
continuing to write the story of Mermaid at GeoAzur near Nice,
with ERC funding.
In 2012 we acquired $83,000 in support from the A. H. Phillips Fund at
Princeton University. In 2013 we acquired $45,001 in support from the
U.
S. National Foundation. With this we built and tested the very
first prototype. The Son-O-Mermaid
instrument website is live!